1. Field of Invention
The present invention relates to a combination of systems and methods for converting agricultural waste, such as animal manures, to alcohols and methyl esters, with the concurrent generation of emission-free electricity and reuse of the wastewater produced. The systems and methods of the invention provide efficient and sanitary disposal of agricultural waste and produce useable energy and fertilizer.
2. Description of Related Art
Small, family farms are steadily being replaced by larger integrated confinement corporate agricultural operations. Such large agricultural operations often utilize confinement barns for concentrated housing of large numbers of livestock units, such as swine, dairy cattle, or poultry. The swine and poultry industry have tended to concentrate such operations in a few counties nationwide. Some individual counties may contain greater than one million hogs. The meat production economies of scale enjoyed by such corporate farmers are offset by the pollution problems arising from the handling and treatment of the collected waste and other manures from such operations. Not only do the collected agricultural waste streams have concentrations of nitrogen, phosphorus, bacteria and pathogens, but also noxious odors are a standard of current operations. In addition, there is a high potential for releases from the wastewater containment lagoons during periods of excessive rainfall, greatly endangering surrounding waterways and neighboring properties.
Currently such concentrated farming operations treat their wastewater with a combination of aerobic and anaerobic digestion in open lagoons due to its simplicity and lower capital costs. In this system the raw wastewater is simply discharged from the confined animal facility into open lagoons where the waste undergoes natural anaerobic digestion, followed by aerobic polishing. During this process greater than forty noxious gases, including ammonia, hydrogen sulfide and methane, are emitted to the atmosphere in an unregulated fashion. Certain regulations require a residence time for such wastewater in an open lagoon of 180 days for more complete digestion. Adjacent properties suffer from both the odors and the runoff of the sprayfield discharge of the treated effluent. In addition, studies have shown that lagoons leak causing pollution problems in groundwater, rivers, estuaries and lakes in areas of high concentration of confined animal farming.
In the case of swine manure treated in standard anaerobic lagoons, the liquid effluent has nutrient characteristics including high levels of biological oxygen demand (BOD5), (total amount of biological oxygen demand measure over a five day period). Total Nitrogen (TN) and Phosphorus (P) than can be reduced to permitted levels by anaerobic digestion alone. Even with bacterial digestion, significant amounts of sludge accumulation in an anaerobic lagoon may cause it to reach capacity quickly and fail to provide long term effective wastewater treatment.
Swine produce two to four times as much waste, per hog, as the average human. In North Carolina alone, hogs produce about 9.5 million tons of manure per year. Each swine producing farm utilizes a great deal of land for spreading the highly concentrated wastewater following digestion as very often no discharge is permitted from animal waste facilities. Regulations require farms to utilize increasingly large areas as sprayfields for this effluent as residual concentrations of nutrients may cause permanent damage to the soil and groundwater in these areas. Some states have contemplated a moratorium on new concentrated farming facilities due to the agricultural wastewater treatment problems. In North Carolina, owners of existing agricultural wastewater treatment facilities constructed prior to 31 Dec. 1993, must register with the state and have their animal waste plan certified by a third party.
The initial application of anaerobic digestion of organic waste streams was the septic tank, invented in 1895. Beginning in the 1950's, anaerobic digestion processes were applied to livestock slaughtering wastewaters. This anaerobic contact process made use of a reactor tank in which contact between anaerobic microorganisms and wastewater occurs. The wastewater then flows through a vacuum degasifier to remove dissolved gas to enhance solids settling and then on to a separate solids separation unit in which biomass solids settle to the bottom for recycling to the contact tank. In these existing systems, multiple vessels are required which have a high capital investment, and there are inherent inefficiencies in multi-step processes. Additionally, the biogas produced in such systems typically is released to the atmosphere.
U.S. Pat. No. 4,372,856 discloses a method and apparatus for anaerobic digestion of agricultural waste to produce biogas containing mainly methane and carbon dioxide. The method utilizes ammonia generated during the anaerobic digestion to strip hydrogen sulfide from the biogas containing methane and carbon dioxide. U.S. Pat. No. 5,922,092 discloses a gasification system whereby solid wastes, including agricultural waste, are contacted with gas in a thermal reactor. A by-product of this method includes diesel fuel, asphaltic materials, liquified gases and others. U.S. Pat. No. 6,410,283 discloses a method for generating electricity by anaerobic digestion of sewage sludge. The disclosures of these patents are incorporated by reference herein in their entirety.
Methane-rich gases have been converted to synthesis gases by reformation to the basic elements of such feedstock for many years. The reformation of methane-rich gases at elevated temperatures in the presence of steam and a heterogeneous catalyst also has been accomplished for a number of years. The efficacy of such processes has enhanced by the introduction of Carbon Dioxide (CO2) from external sources to the methane-rich gas stream. The addition of more carbon atoms from external resources serves to create a more balanced Hydrogen (H2) and Carbon Monoxide (CO) product of the reforming process.
Converting synthesis gas to methanol by passing the synthesis gas over a heterogeneous catalyst at controlled pressure and temperature has been accomplished on a commercial scale for a number of years. The mixed alcohol product of this process is used primarily as feedstock for the production of formaldehyde or methyl tert-butyl ether (MTBE). Producing methyl esters from mixed alcohols in combination with a catalyst and fats or oils under heat and pressure has been accomplished for a number of years on a commercial scale. Such products also do not qualify as a fully renewable product.
Methods of treating wastewater rich in nutrients are disclosed, for example, in U.S. Pat. No. 626,644 to Northrop, U.S. Pat. No. 4,721,569 to Northrop, U.S. Pat. No. 4,183,807 to Yoshizawa, et a.l and U.S. Pat. No. 5,185,079 to Dague. Methods of utilizing agricultural waste or biomass as fuel for electrical generation are disclosed, for example, in U.S. Pat. No. 5,121,600 to Sanders, et al. Methods of converting methanol and fats or oils to methyl esters and biodiesel are disclosed in, for example, U.S. Pat. Nos. 5,713,965 to Foglia, et al., 6,015,440 to Noureddini, and 6,440,057 to Nurhan, et al. The disclosures of these patents are incorporated by reference herein in their entirety.
Notwithstanding the existence of these known treatment and conversion systems, there is a need for an agricultural wastewater treatment system that is capable of digestion of a higher organic solid fraction and conversion of the produced biogas to a transportable, liquid energy product on a continuous basis. Efforts to improve extant methods and apparatus continue.
The description herein of disadvantages associated with known processes and apparatus is by no means intended to limit the scope of the invention to embodiments that exclude the known processes and apparatus. Indeed, certain embodiments of the invention may include some or all aspects of known processes and apparatus, in some instances without suffering from the above-described disadvantages.